<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>

	<button id="btn">btn...</button>
	
	<script>
		// w3c
		var btn = document.getElementById('btn');
		/*btn.addEventListener('click', function() {
			alert(999);
		}, false);*/
		// removerEventListener();

		// IE
		btn.attachEvent('onclick', function() {
			alert(888);
		});
		// detachEvent
	</script>
</body>
</html>